In digital communication or broadcasting, the transmission end transmits information in the form of electromagnetic wave through a physical channel, such as air, to the receiving end. Due to the non-ideal channel effect, such as multi-path reflection and signal fading, there is usually distortion at the receiving end. Because the OFDM based on the multi-carrier modulation is effective in handling the multi-path reflection problem, it is becoming a mainstream technology in the wire/wireless communications and digital broadcasting-terrestrial systems.
The OFDM applications include: asymmetric digital subscriber line (ADSL), very-high-speed digital subscriber line (VDSL), digital audio broadcasting (DAB), wireless local area network (WLAN) IEEE802.11a/g/n, Ultra Wideband (UWB), dedicated short range communication (DSRC), integrated service digital broadcasting-terrestrial (ISDB-T), digital video broadcasting-terrestrial broadcasting (DVB-T), and digital video broadcasting-handheld (DVB-H), and so on.
The output signals from the OFDM transmission end will have a guard interval with cyclic prefix placed in front of each symbol to fight against the inter-symbol interference (ISI) caused by the multi-path effect. FIG. 1 shows a complete OFDM symbol, in which the guard interval includes Ng sample points, and the original symbol includes N sample points; therefore the length of the symbol is N+Ng. The European TV transmission defines the 2K mode and the 8K mode of the original symbol length, combined with four different guard interval lengths, including 1/32, 1/16, ⅛, and ¼ of the original symbol length for protection in different transmission environments.
Before processing the data, the OFDM receiving end must remove the data of the guard interval interfered by the ISI in order to extract correct effective data range to perform the Fast Fourier Transform (FFT) of the corresponding length (2K or 8K mode). If the extracted data range is interfered by ISI, the following data will all be affected by a phase difference. This may prolong the time required for decoding the symbol, or even corrupt the following data beyond decoding. It is therefore important to provide an effective symbol synchronization method.
The current symbol synchronization methods are divided into three categories. The first category includes methods, based on the repetition characteristics of the guard interval data, using correlation computation to analyze the correlation magnitude for processing. The second category includes methods, based on the repetition characteristics of the guard interval data, using correlation computation to analyze the correlation phase for processing. The third category includes methods combining the above two characteristics.
U.S. Pat. No. 6,421,401 disclosed a method based on cyclic prefix of the guard interval, combing delay conjugate multiplication and moving average techniques to process the data with two passes of delay correlation computation and observe the magnitude of the generated characteristics signal to find the maximum position as the output. This method analyzes the correlation magnitude for processing.
U.S. Pat. No. 5,991,289 disclosed a method based on cyclic prefix of the guard interval, combing delay conjugate multiplication and moving average techniques to process the data with delay multiplication to extract the phase difference and a moving sum to generate the characteristics signal, and then observe the transition position of the phase difference as the output. This method has the drawback of being easily affected by the channel decay or noise to misjudge the synchronization position.
US. Patent publication No. 2004/0,208,269 disclosed a method using correlation magnitude and correlation phase. After extracting, the correlation magnitude is multiplied with the correlation phase, passed through a self-parameter filter, and computed with a delay correlation computation, the transition position of the characteristics signal is observed as output. This method has the drawback of high complexity.